Annotations are independent markings, notes and other miscellaneous information such as highlighting that is added to a document after the document (or at least part of it) is in some state in which it can be reviewed. Note that annotating is not the same as re-editing one draft document into a later version, but instead refers to adding material that is generally independent in meaning and semantics, e.g., something a reviewer previously would have done on a paper copy printout of the document. Today, annotating of electronic documents may be done by certain computer programs, such as word processing programs.
While some computer programs thus allow annotations to be electronically added to documents created by that program, there are many types of documents, programs and content formats that exist in contemporary computing. The programs that allow the addition of such annotations typically do so by inserting the annotations within the content, that is, directly into the content in a proprietary format. The annotations may be rendered according to the program, e.g., as footnotes, bubbles, superimposed handwritten notes, and so forth. Because the annotations are within the content, the ability to use a computer system to electronically annotate content and read back the annotations essentially depends on the program that created the content, or at least a similar program that understands the format.